EP0312428A1 - Apparatus for injecting a hydrocarbon feed into a catalytic cracking reactor - Google Patents

Abstract

Device for injecting a hydrocarbon feedstock into a catalytic cracking reactor, comprising, from upstream towards downstream in the direction of travel of the feedstock, a system (5) for allowing to enter and mixing the liquid hydrocarbon feedstock and steam, a venturi (6) whose converging cone (10) is connected to the said system (5) for allowing to enter and mixing, and a protective hood (7) integrally attached to the diverging cone (14) of this venturi and having an orifice (16) for injecting the feedstock into the reactor. <??>The dimensions of the venturi (6) are such that the speed of mixing of the liquid feedstock and of steam therein reaches sonic conditions at the neck (13) and the rectilinear neck situated between the divergent cone (14) and the convergent cone (10) is connected to the latter in a continuous manner by a curved profile in the form of an angle greater than 5 DEG and approximately 15 DEG respectively with the axis of the venturi. <IMAGE>

Description

The present invention relates to a device for injecting a hydrocarbon feedstock into a catalytic cracking reactor, in particular in a fluidized bed.

In the processes of catalytic cracking in a fluidized bed, called in the technique FCC processes, the hydrocarbon charge is injected into a reactor in the form of a column, in which the catalyst is kept in suspension and moves either in an essentially ascending flow ( the reactor is then called "riser"), or in a downward flow (the reactor is then called "dropper").

Reference will be made below, for the sake of clarity, to the case of upward flow reactors, but the transposition to downward flow reactors will be obvious to those skilled in the art and the charge injectors which are the subject of the invention. present invention apply of course to both types of reactors.

In a riser, the regenerated catalyst is introduced at the base of the reactor at the same time as a carrier gas, in a fluidized and hot state (between 650 and 850 ° C.), below the injection zone of the hydrocarbon charge. This is introduced into the reactor in an essentially liquid state and at a temperature between 80 and 300 ° C.

Between the charge injection zone and the top of the reactor, the catalyst yields part of its heat energy to the charge, which is then vaporized and cracked into light hydrocarbons. This has the effect of rapidly increasing the volume of gases, which transport the catalyst accelerated to the top of the reactor, where it is separated from the hydrocarbons. The mixture of hydrocarbons and grains of catalysts reaches, at the outlet of the riser, an equilibrium temperature usually between 470 and 530 ° C.

During these operations, a small part of the charge (generally between 3 and 12% by weight) has formed on the catalyst particles a solid hydrocarbon or "coke" deposit, which reduces the catalytic activity of the catalyst and limits the conversion of the charge into recoverable products. It is therefore necessary to regenerate the catalyst by combustion of this deposit of coke, before reintroducing it into the reactor for a new cracking cycle.

The deposition of coke on the catalyst is generally all the more important as the charge injected is heavier and, consequently, more difficult to vaporize. The non-vaporized fractions then form on the catalyst a film mainly composed of coke and heavy hydrocarbons.

In order to limit this phenomenon, it is therefore important to avoid liquid fractions of the hydrocarbon charge coming into contact with the catalyst before being vaporized, then cracked, and it is therefore necessary to reduce the vaporization time of the charge.

Furthermore, the advantage of injecting the feedstock to be treated at high speed and in the form of very fine droplets into the reactor is well known in the art (see US Patents 2,891,000 and 2,994,659).

It has been proposed to use for this purpose charge injectors comprising a venturi (see patents FR 2 102 216, US 3 240 253 and US 4 523 987; see also EP-A-157 691), but this technique poses many problems with injection into fluidized bed reactors. In fact, the velocities at the level of the venturi neck cause the veins of fluids to detach from the walls of the injector, the formation of recirculation currents and penetration into the injector of catalyst particles, with consequent marked erosion of the injection device and rapid deterioration in performance.

Impaction injection devices have also been proposed (see US Pat. No. 4,434,049), which use mechanical means to atomize the charge, but the quality of the spraying of the charge, at equal energy, is much less good.

The present invention aims to remedy these drawbacks by proposing a device for injecting a charge of hydrocarbons in a catalytic cracking reactor which causes only a small drop in injection pressure, which atomizes the charge into very fine droplets and injects it at high speed into the reactor without risk of erosion of the injector, or catalyst attrition downstream of the injector.

To this end, the subject of the invention is a device for injecting a hydrocarbon charge into a catalytic cracking reactor, comprising, from upstream to downstream in the direction of displacement of the hydrocarbon charge, a system for admitting and mixing the liquid charge of hydrocarbons and water vapor, a venturi the convergent of which is connected to said intake and mixing system, and a protective cover integral with the diverging portion of this venturi and having a orifice for injecting the charge into the reactor, this device being characterized in that said venturi has dimensions such that the speed of mixing of the liquid charge and the water vapor reaches there sonic conditions at the level of the neck, in that the rectilinear neck, located between the divergent and the convergent, is connected thereto in a continuous manner by a curvilinear profile, without forming an angle greater than about 5 ° and about 15 ° respectively with the axis of the venturi, and in that the surface of the injection opening of the protective cover is between 1.5 and 10 and preferably between 2 and 5 times that of the neck of the venturi.

As will be seen below, this combination of characteristics has the effect of atomizing the charge into droplets of very small diameter, which can be of the order of 30 microns on average, and of allowing injection into the reactor at very high speeds, up to 150m / s, without risking, however, significant introduction of catalyst grains by return to the venturi, which would result in high erosion and a limited lifetime thereof.

In particular, the presence of a protective cover at the end of the divergent, with a reduced injection orifice, makes it possible to limit the risks of penetration grains of catalyst in the injection device. Furthermore, the curvilinear profile of the venturi and the limitation of the angles of the divergent and convergent to the indicated values prevent the veins of fluid from coming off. If grains of catalyst enter the device, they are therefore not entrained by recirculation currents, but on the contrary are immediately returned with the charge to the reactor.

The injection device according to the invention is therefore distinguished by the following advantages of the known devices:
- increased longevity, due to less wear due to the grains of catalyst;
better efficiency, since the droplets of the feed have a diameter of between approximately 10 and approximately 50 μ and are injected at speeds of between approximately 60 m / s and 150 m / s;
- a relatively low pressure drop at the injector;
- negligible clogging risks, due to the sonic conditions leading to a high speed at the level of the venturi neck;
- An almost instantaneous vaporization of the charge in the reactor, with the consequences of a better cracking efficiency and a reduction in the coke deposits on the catalyst particles.

The charge may be introduced in the form of a jet into the reactor, generally in the direction of displacement of the catalyst, at an angle of 20 to 40 ° with this direction.

The mixture of the catalyst in a fluidized bed and of the charge injected will be made all the more easily and intimately as the speed of injection of the charge will be higher. With the injection device according to the invention, this injection speed will be much higher than that of the usual methods of catalytic cracking in a fluidized bed. Due to the small diameter of the droplets of hydrocarbons, heat transfers and vaporization will be almost instantaneous. In addition, it is thus possible to better control the duration of contact between the catalyst and the vaporized hydrocarbons, which is important, when it is desired to limit this duration to a few seconds, or even to a few fractions of a second.

The injection of the charge into the reactor will preferably be carried out using several injectors arranged at the same level and distributed regularly at the periphery of the reactor. We can use, for example, from 2 to 12 charge injectors, depending on the characteristics of the cracked reactor in a fluidized bed.

The shape of the orifice of the protective cover of the device will be determined by the geometrical conditions of the reactor, by the number of injectors, their arrangement, etc. This orifice may be, depending on the case, circular, oval, or in the form of a slot. This protective cover may advantageously have the form of a spherical cap and the orifice may, in a manner known per se, be provided with lips intended to shape the shape of the jet of charge to be cracked sprayed into the reactor. This orifice may even, on occasion, consist of one or more tubes whose total section will therefore necessarily be between 1.5 and 10 times that of the venturi neck. According to one of the preferred forms, but in no way limiting, of the invention, the droplet outlet orifice may be in the form of a simple slot arranged in a plane of symmetry of this cap and the injection into the The reactor can then take the form of a triangular flat jet which will contribute to the rapid and uniform contacting of the charge to be cracked and the catalyst. The opening angle of this jet can advantageously be between 60 and 90 ° C.

As indicated above, the injection device according to the invention will comprise an intake and mixing system for the charge of hydrocarbons and atomizing gas, which may advantageously be steam d water or any other acceptable gas phase. This system will include, for example, two concentric tubes, the central tube being able to bring the water vapor and the peripheral annular space being able to be used as passage to the load or the reverse according to the cases, the external tube being connected to the venturi, while the central tube can open out upstream or level of the venturi convergent.

In order to avoid any separation of the walls of the mixture of the charge and the water vapor and thus to eliminate any harmful penetration of the catalyst into the charge injector, the venturi will have no sharp edges and the convergent and the diverging of it will be connected to the neck with continuous curvilinear profiles having a radius of curvature of 2 to 3 meters. The divergent will generally make an angle of about 3 degrees with the axis and the convergent an angle of about 12 degrees.

The rectilinear neck of the venturi will have a diameter such, taking into account the conditions of supply of superheated steam and of hydrocarbon charge (flow and pressure) that the injector operates in critical regime, that is to say that the overall mass flow no longer depends on the pressure difference between upstream and downstream of venturi, but only on the upstream pressure, This gives an atomization of the liquid charge into very fine droplets, the diameter of which can be as small as 30 µ , against 500 µ with the traditional system. All other conditions being equal, experience shows that droplets of very small diameter can vaporize in the reactor in less than 3 milliseconds approximately, whereas, under identical conditions, approximately 230 milliseconds are necessary for the droplets of 500 microns of usual injectors. The mass and heat transfer zone is then reduced from 150 to 3 centimeters and it is clear that the risk of liquid fractions being deposited on the catalyst is reduced all the more, before their vaporization or cracking.

Experience also shows that the diameter of the droplets obtained at the outlet of the venturi varies in opposite direction to the ratio between the mass flow rate of steam and the smallest passage section S _c (neck) of the venturi.

Atomi liquid and gas charge rates With the station fixed, the neck diameter will be calculated by the relationship between the upstream pressure and the mass flow of liquid and gas at the neck.

The nominal flow rate of the load is normally fixed by external requirements (process characteristics, capacity of other equipment in the unit, etc.) and the flow rate of atomizing vapor therefore depends on that of the load.

The diameter of the downstream end of the diverging part of the venturi will be determined in such a way that the speed of the vapor + hydrocarbon mixture is between 30 and 150 m / s and, preferably, between 30 and 120 m / s, so as to avoid attrition of the catalyst in suspension by the jet exiting the cover, all the more so that the latter accelerates the mixture on leaving the diverging portion.

The length of the venturi neck will generally be between 1 and 10 times its diameter.

Other dimensional characteristics of the venturi, of the supply and mixing system and of the cover will appear in the detailed description which will be given below of an injector according to the invention.

This injector can equip both a new catalytic cracking reactor in a fluidized bed and an old renovated unit. On these refurbished units, the pressures available for charging and for steam are lower than in the new units, but very satisfactory performance (drop diameter less than 120 microns) can still be obtained.

The injector according to the invention is fixed in the usual way to the wall of the reactor by means of a sleeve, with the interposition of a thermal insulator.

The accompanying drawings illustrate an embodiment of such an injector. They are not limiting.

• Le figure 1 est une coupe schématique illustrant le montage de l'injecteur conforme à l'invention sur la paroi du réacteur;
• La figure 2 est une vue en coupe de l'injec­teur et de son système d'alimentation et de mélange.

In these drawings:

• Figure 1 is a schematic section illustrating the mounting of the injector according to the invention on the wall of the reactor;
• Figure 2 is a sectional view of the injector and its supply and mixing system.

The charge injector shown in FIG. 1, designated by the general reference 1, crosses the wall 2 of the reactor and is fixed thereon using a sleeve 3, with a thermal insulator 4.

The injector 1 essentially comprises three parts, namely:
- a feeding and mixing system 5;
- a venturi 6 or any other convergent-neck-divergent assembly;
- a cover 7.

The supply and mixing system, 5, illustrated in FIG. 2, comprises two concentric tubes, a central tube 8, used for conveying the liquid load of hydrocarbons to be treated, and a second external tube 9, supplied laterally at 10 in superheated steam, which circulates between tubes 8 and 9.

This mixing system has a double purpose:
- ensure a mixture of charge and steam, which minimizes swirls and pressure drops;
- Perform this mixing as close as possible to the convergent 10 of the venturi 6 connected to this system, in order to avoid possible problems of condensation of the vapor on contact with the cooler load.

The charge introduction tube 8 enters the venturi 6 and opens immediately upstream of the convergent 10.

The tube 9 and the venturi 6 are assembled by flanges, 11 and 12 respectively. Fins 18 ensure the centering of the tube 8 at the level of the flange 11.

When sizing tubes 8 and 9, the speed of the hydrocarbon charge must be taken into account, generally between 1 and 10 m / s. and that of steam, between 10 and 80 m / s.

The venturi 6, comprising the convergent 10, a straight neck 13 and a divergent 14, constitutes an important part of the injector according to the invention. He has the purpose, in fact, of accelerating the liquid charge mixed with the superheated steam to a pseudo-critical speed at the level of the neck 13, to cause its fragmentation of very fine droplets with a diameter of less than about 120 microns and with an average diameter between 30 and 50 microns. Furthermore, it must allow the mixture formed of these droplets and water vapor to have a speed of between 30 and 150 m / s at the outlet of the divergent 14.

Preferably, the length x of the straight neck will be substantially equal to five times the diameter d of this neck.

The convergent 10 and the divergent 14 will raddord to the neck 13 continuously by a continuous curvilinear profile, of 2 to 3 meters in radius, without discontinuities likely to cause detachment of the fluid vein from the internal walls of the venturi, which would have effect of causing vortices favoring the entry of particles of the catalyst, with as a consequence an erosion of the walls of the divergent 14.

The angle of the convergent 10 will be generally of the order of 10 to 15 °, and the angle of the diverging 14 will be generally of the order of 2 to 5 °.

At the end of the diverging part is fixed a hemispherical cover 7, having here, in a diametrical plane, a slot 16 intended to inject into the reactor the charge of droplets of hydrocarbons in the form of either a cone or a substantially flat brush. The angle of the jet coming from the injector will naturally depend, in a manner known per se, on the geometry of the slot and of the injection zone, as well as on the operating conditions of the reactor and on the number of injectors used.

The speed of exit of the cover of the hydrocarbon droplets will preferably be between 30 and 120 m / s and these droplets will preferably be injected into the reactor so as to cover the cross section thereof as uniformly as possible. Generally used in a manner known per se from 2 to 12 injectors arranged at the same level and directed in the direction of movement of the catalyst bed (arrow F in FIG. 1), at an angle of about 20 to 90 ° with the axis of the reactor.

The high injection speed of the load and the very small size of the hydrocarbon droplets facilitate their complete vaporization in a few milliseconds.

Claims (9)

1.- Device for injecting a hydrocarbon charge into a catalytic cracking reactor comprising, from upstream to downstream in the direction of displacement of the charge, a system (5) for admitting and mixing the charge hydrocarbon liquid and water vapor, a venturi (6) whose convergent (10) is connected to said intake and mixing system (5), and a protective cover (7) integral with the divergent (14) of this venturi and having an orifice (16) for injecting the charge into the reactor, this device being characterized in that said venturi (6) has dimensions such that the speed of mixing of the liquid charge and the vapor d water reaches sonic conditions at the level of the neck (13), and the rectilinear neck situated between the diverging and the converging (10) is connected thereto in a continuous manner by a curvilinear profile without forming a higher angle respectively about 5 ° and about 15 ° with the axis of the venturi. The surface of the injection orifice (16) of the protective cover (7) is between 1.5 and 10 and preferably between 2 and 5 times that of the neck of the venturi. 2.- Device according to claim 1, characterized in that said intake and mixing system comprises two concentric tubes, one which can be connected to the supply of hydrocarbons and the other which can be connected to the supply of water vapour. 3.- Device according to claim 2, characterized in that said internal tube (8) opens upstream of the convergent (10) of said venturi (6). 4.- Device according to one of claims 1 to 3, characterized in that said venturi (5) has, in axial section, no sharp edge, the connecting parts of the convergent (10), of the neck (13) and divergent (14) having a radius of curvature of 2 to 3 meters, in that the convergent (10) of said venturi makes with the axis thereof an angle (α) of between about 10 and 15 °, and in what the divergent (14) of said venturi makes with the axis thereof an angle (β) between about 2 and 5 °. 5.- Device according to one of claims 1 to 4, characterized in that the length of the neck (13) of said venturi, is equal to about 1 to 10 times its diameter. 6.- Device according to one of claims 1 to 5, characterized in that the diameter of the downstream end of the diverging portion (14) of said venturi is such that the speed of the mixture of water vapor and hydrocarbon droplets is between about 30 and 150 m / s. 7.- Device according to one of claims 1 to 6, characterized in that said cover (7) has the shape of a spherical cap and in that the injection orifice (16) of said cover has the shape of 'A slot arranged along a diametrical plane of said cover. 8.- Device according to claim 7, characterized in that said slot occupies a circular sector of said cover between 60 and 90 °. 9.- Device according to one of claims 1 to 8, characterized in that the axis of the injector makes an angle of 20 to 90 ° relative to that of the reactor.

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